Let me give one example of a distinction between walking in the Spirit of Christ versus walking in that of the world. It is only an example to make a point.
Imagine you are holding a broom. Sweeping the floor around your house. Under tables, around chairs. Pulling dirt together into one large pile. As you sweep paper, dust, and other things out from under a table, you notice a tiny dust spider. The kind that looks very fragile and almost transparent. Hardly anything intimidating. It walks gracefully and honest, and without threat. You notice it for a split second. But you simply sweep it up with the rest of the trash. Something you have probably done many times before.
Now think about the same scenario. But instead, it is a small child doing the sweeping. They come across the spider. Would they simply sweep it up into the pile? Not likely so. They would certainly acknowledge it. They would either get scared and run away. Even though its the least harmful spider you can think of. Or they would feel sorry for it. In their mind, they would momentarily forget the trash pile. Instead, worrying more about the life of the spider.
So what has happened between the time we were a small child to the time we became insensitive about a single life-form? As insignificant as that life may seem to us, is it really insignificant? As crazy as this may sound to some people. Every life that GOD creates is significant. Would He bother to create anything at all that isn't significant? When we think about the Spirit of the child, we begin to see what Christ and His apostles were talking about. A child typically does not see the world the way we do. Their hearts have yet to be corrupted by the world. But they do have foolish hearts, ready to be corrupted by something. They come into this world, ready to have their hearts filled with the LORD. Because if that does not happen, then their hearts will most certainly be filled with corruption. We must remove ourselves from the world and obtain the heart of a child. Not foolishness. But purity. We often find ourselves becoming adults, finding out that we have already lived such a long life of ignorance and Sin. There is much to be undone. And there is no way we can remove all that by ourselves. We don't have the power to do so. We MUST be led by the Holy Spirit to do so. Our hearts must betray the things that are in it. It must be emptied. The lusts. The passions. The pride. The blessing is that those things require constant renewal to be there at all. It is why society is designed to constantly pour Sin into our hearts, over and over again. We must remove ourselves from the ways of the world. Protecting our hearts is not an easy thing to do. We don't have the clear sight to fully know what that entails. Because we have been led to think the "feel goodness" of pride is a good thing. But it is not. And that is why only the Holy Spirit can teach us otherwise. Because He is within us, guiding our conscience. Protecting us from ourselves and the world. Keeping us on the path to purity. Allowing our hearts and our spirit to change. Does this sound like a large task? Nowhere near as large as the reward. The world has nothing at all to offer us in comparison.